First time poster. When I turn on the hot water from the bathroom sink it runs cold. When I turn on the bathtub to hot, the sink will then run hot. When I turn off the bathtub, the sink returns to cold. The faucet is a single handle. We have a tankless water heater and all other hot water taps seem to work. Any ideas on what's happening?

Did this just start suddenly? Any plumbing work done lately? What's the brand name of your tub valve? Does it have Temp Control or Anti Skald ? Any other details you can give? Regards, tom

Thanks for your help Tom. All seemed well until we returned from a two-week vacation. We had some work done about a year ago... sewer line replaced from back of house to street, branch to bathroom tub and sink replaced. No anti-scald or temp control. We live in a single-family, one-story house, two bathrooms. It seems if hot water flows hot from the tub it show flow hot from the sink. Why would running the tub make a difference?

RH


Sorry... not sure of tun valve brand.

RH

You have a crossconnnection, my friend!
Since you haven't has any work done that narrows the field to either the lavatory cartridge or the tub cartridge. That's the only place a crossconnection could happen. I may not know what is is but I certainly know where it is. Good luck, Tom

Thanks again Tom. What is a cross connection and how do you think it is related to my problem? What needs to be done... could I do it myself or should I call a plumber?

RH

Thanks again Tom. What is a cross connection and how do you think it is related to my problem? What needs to be done... could I do it myself or should I call a plumber?

RH

A crossconnection is where one system, (in this case hot and cold) is connected by accident to another resulting in the two commingling. The only place hot and cold come together is in your one handled faucets and the only two faucets affected are the lavatory and tub. Soooo,
I would start with the tub valve. Shut the water off at the integral tub stops located behind the face plate or in a access panel behind the front of the tub and open up the valve to remove the cartridge. Check it over for trash in the inlet portd or a broken "O" ring. Now with the tub faucet shut down see how the lavatory faucet works now. Let me know. Good luck, Tom